The present invention is generally related to measuring methods and systems. The present invention is also related to optical measuring methods and systems. In addition, the present invention is related to methods and systems for measuring the angular displacement and relative torque between two rotating shafts. The present invention is additionally related to non-invasive optical measuring techniques. The present invention is also related to optically sensing techniques for measuring relative mechanical characteristics between rotating members within a mechanical system.
Torque Sensors
A variety of techniques for measuring torque in mechanical systems have been attempted. To date, however, none of these techniques have been completely satisfactory. Several methods of measuring torque within a shaft, strain or optical gauge have been described in the literature. As explained herein, such measuring techniques are generally limited in their scope and applications and are inherently unreliable in both their efficiency and accuracy.
Torque can be measured in a shaft by bonding strain gauges in a cross arrangement along helical lines of compression and tension. The strain gauges can be electronically configured via a balance-bridge and coupled to measuring electronics through slip rings or non-contacting rotary transformers. Generally, these cross arrangements are difficult to implement and usually require custom installation.
In optical torque transducers, light beams, code patterns and light sensors convert the differential angular displacement between two positions on a shaft into an output signal, due to applied torque. Specifically, identical patterns made of light reflecting strips can be arranged around the circumference of the shaft at two locations. The patterns may be illuminated by laser diodes and the reflected light sensed by a photocell. The output of each photocell can be configured as a pulse train wherein the phase difference is a measure of the torque. In a similar device, which is taught by Kawamoto, U.S. Pat. No. 4,767,925, Optical Type Relative Rotation Measurement Apparatus, a pair of light emitting and receiving elements produces an output dependent on the amount of light transmitted due to the relative rotation of two slotted disks. Levine, U.S. Pat. No. 4,433,585, Device for Measurement of the Torsional Angular Deviation of a Loaded Rotating or Static Shaft, discloses a technique for passing a beam of light through two diffraction gratings placed at different locations along a shaft and sensing the phase of the two resulting beams. Such techniques and devices thereof are not robust because they require precise alignment for optimal functioning.
U.S. Pat. No. 5,001,937, Optically Based Torsion Sensor to Bechtel et al., discloses an optically based torsion sensor that measures the phase displacement between two bands of alternating high and low reflectivity regions. A major drawback of this device is its dependence on the initial alignment of the two bands. In addition, minor differences in the rise time of detecting electronics will cause serious errors in measurement. U.S. Pat. No. 4,525,068, Torque Measurement Method and Apparatus to Mannava et al., discloses a torque sensor utilizing optical Doppler measurements. Since Doppler measures velocity only, this device suffers from a serious shortcoming in that it must infer torque from changes in instantaneous rotational velocity of two different sections of a shaft.
Two optical methods for measuring the strain of an object are noteworthy. U.S. Pat. No. 4,939,368, Polychromatic Optical Strain Gauge, to Brown, discloses an optical grating to measure strain in a stationary object. The device is complicated in that it requires two frequencies of light and has no provision for measuring a moving object such as a rotating shaft. U.S. Pat. No. 4,432,239, Apparatus for Measuring Deformation, to Bykov, discloses an apparatus for measuring the deformation of an object. The device utilizes an electro-optical frequency modulator to produce two components from an incident laser beam. A polarization splitter further splits the light into two different frequencies, which illuminate a diffraction grating on a stationary object. This device is also complicated and expensive and has no provision for measurement of a moving object such as a rotating shaft.
The literature discloses a capacitive torque sensor consisting of two encoders either mounted perpendicular to the shaft at each end or mounted along the circumference at two closely placed points along the shaft. For example, see Interest in Misfire Detection Technology Grows, Automotive Electronics Journal, Nov. 6, 1989, pg 12. Each encoder has two parts: a stator that consists of up to 256 radial fingers that are alternately charged; and a rotor that is generally mounted on the shaft. As the shaft turns, the rotor""s potential switches between positive and negative at a frequency proportional to speed. A disk, at the center of the stator, electrically isolated from the charged fingers, collects the signal. Like the optical torque sensor, the twist of the shaft is determined by measuring the phase difference between the two encoders. Also, like the optical sensor, this device requires precise alignment.
Finally, magnetic torque sensors comprise much of the prior art. The magnetic properties of most ferromagnetic materials change with the application of stress to such an extent that stress may be ranked with field strength and temperature as one of the primary factors affecting magnetic charge. Magnetostriction is a measure of the stress sensitivity of a material""s magnetic properties. Magnetic-based torque sensors take advantage of the magnetostrictive properties of ferromagnetic metals, such as carbon steel. See Noncontact Magnetic Torque Transducer, Sensors, November 1990, pp. 37-40. These sensors make a contact-less measurement of changes of magnetic permeability in shaft materials, which are caused by torsional stress.
In place of strain gauges, magnetic flux is directed into the shaft and along the helical lines of compression and tension. A positive magnetostriction shaft experiencing torsion will exhibit increased permeability along the line of tension and decreased permeability along the line of compression. At low stress levels the permeability is nearly linear with stress but varies dramatically at high stress. Another drawback of a magnetostrictive torque sensor is in the need for calibrating it individually with each shaft. This requirement is obvious because the torque measurement is made by means of the magnetostrictive properties of the shaft material and cannot be predetermined in the manufacture of the sensor by itself.
The variability in magnetostrictive properties is usually correlated with the variability of the mechanical hardness of the material. Hardness variability of shaft materials typically ranges from +10% to +40%. The shaft-to-shaft variability problem has been addressed in recent research by adding either a sleeve or coating of a well-defined and magnetically soft material, such as nickel, permalloy, or ferromagnetic amorphous alloys. While this approach shows promise, installation can not be made in situ, and all magnetic materials, even the softest, can retain some magnetism, leading to non-linearities and drift.
Each of the above-mentioned techniques for measuring torque falls short of the ideal due to a variety of shortcomings, which include high cost, inadequate resolution and sensitivity, extreme dependence on precise alignment, inability to be applied in situ, or susceptibility to environmental conditions. Therefore, there exists a need for an economical, accurate, simple, non-contact sensor for the measurement of relative torque.
Moirxc3xa8 Fringes and Talbot Self-Image Effect
When electromagnetic rays, including light rays, are impinged and reflected off a pattern contained within an encoded surface, such as a diffraction grating, a bar code or a grooved surface, the reflected rays form an image of the pattern of the encoded surface. When such reflected electromagnetic rays from two such encoded surfaces are subsequently made to overlap, or interact, the resulting pattern is a super-position of the two simple harmonic functions. In classical optics such patterns are generally known as Talbot interferograms, Fresnel patterns or Moirxc3xa8 fringes. See The Handbook of the Moirxc3xa8 Fringe Technique, K. Patorskii (1993). Similarly, Moirxc3xa8 fringes and Talbot self-image effect could be observed when a single beam of light is transmitted through two gratings, or bar codes, placed serially with respect to one another. For detailed mathematical theory on the formation and the Talbot self-image effect see Introduction to Fourier Optics, by J. W. Goodman, 2nd Ed., pgs 87-89.
Moirxc3xa8 fringes and the Talbot self-image effect have hitherto been used effectively in a multitude of applications, including anti-counterfeiting devices, determination of optical characteristics of manufactured lenses, calibration of screen printing devices, etc. A review of the literature reveals that Moirxc3xa8 fringes and the Talbot self-image effect have hitherto not been applied to sensors for the measurement of the relative torque between two rotating shafts.
One aspect of the present invention provides for a measuring method and apparatus.
Another aspect of the present invention provides for an optical measuring method and apparatus.
A further aspect of the present invention provides for a method and apparatus for measuring the angular displacement and relative torque between two rotating shafts.
An additional aspect of the present invention provides optically sensing techniques for measuring relative mechanical characteristics between rotating members within a mechanical system.
An additional aspect of the present invention provides for the utilization of vertical cavity surface-emitting laser (VCSEL) diodes.
The above and other aspects of the present invention are achieved as is now described. An optical angular displacement sensor may be utilized to measure rotary displacement between two shafts that rotate together. By connecting the shafts together utilizing a torsion bar, the sensor may be used to measure transmitted torque. The sensor generally comprises first and second coaxial discs mounted on shafts, which have encoded surfaces adhered (for example, as a vernier) along their circumferential edges. The encoded surfaces are adhered to the disks in such a way that they are parallel to each other. The apparatus operates by reflecting two beams of light over the encoded surfaces adhered to the circumferential edges of the two mounted disks on the rotating shafts.
The light beam may be incident upon the encoded surface of the first rotating disk at a small angle of incidence. Similarly, a second light beam, identical to the first light beam, may be incident upon the encoded surface of the second rotating disk. The image of the encoded pattern from the reflected beam of light from the first disk interacts with the image of the encoded pattern of the second disk as the second beam is reflected off the second encoded surface. By virtue of the design of the encoded surfaces, the reflected beams of light form Moirxc3xa8 fringes that are dynamically stable when the two rotating shafts rotate synchronously.
When the rotating shafts rotate asynchronously, the Moirxc3xa8 fringes exhibit motion that may be detected. The detected signal can be analyzed to yield a variety of relative mechanical characteristics of the rotating shafts, including the relative torque between the shafts. Contrast within the Moirxc3xa8 fringes may be significantly improved by placing the detector at the Talbot distance.
In addition to measuring angular displacement and torque, the apparatus can also be configured to measure performance characteristics such as relative spring and damping coefficients, relative slip and friction, or relative uniformity of motion between the rotating shafts. The apparatus can also be used to determine the direction of rotation and extent of linear motion between the rotating shafts. Finally, the apparatus and method disclosed herein according to the present invention can be utilized to provide feedback to the mechanical system to improve performance and reliability.